1. Field of the Invention
The present invention relates to a multi-beam scanning optical system and an image forming apparatus using it and, more particularly, the invention is suitably applicable to image forming apparatus, for example, such as laser beam printers, digital copiers, and so on, capable of implementing high-quality printing in relatively simple structure and at high speed.
2. Related Background Art
Scanning optical systems used heretofore in the image forming apparatus such as the laser beam printers, the digital copiers, and so on are constructed in such structure that light emitted from a light source is guided to a deflecting means by an incidence optical means, that the light deflected by the deflecting means is focused in a spot shape on a surface of a photosensitive drum, which is a surface to be scanned, by a scanning optical means, and that the surface of the photosensitive drum is optically scanned by the light.
With a recent trend toward higher performance and advanced functions of the image forming apparatus, there is the growing need for higher speed and use of plural light sources is under study in order to meet the need. For example, Japanese Patent Application Laid-Open No. 09-54263 suggests the multi-beam scanning optical system using a multi-beam laser chip as a light source, which is a light source of a single chip for emitting a plurality of laser beams aligned on a straight line.
In the case of such multi-beam scanning optical systems, it is common practice to provide an optical means for detection of synchronism (BD optical system) immediately before writing of image signals in order to accurately control start positions of images.
FIG. 22 is a principal, cross-sectional view in the main scanning direction of a conventional multi-beam scanning optical system (which is a main scanning section view). In the same figure, numeral 51 designates a light source unit, for example, having two light-emitting regions (light sources) of semiconductor laser. The two light-emitting regions are spaced from each other in the main scanning direction and in the sub-scanning direction. Numeral 52 denotes an aperture stop, which shapes each of beams emitted from the respective light-emitting regions, into a desired optimal beam shape. Numeral 53 indicates a collimator lens, which converts the beams having passed through the aperture stop 52, into nearly parallel beams. Numeral 54 represents a cylindrical lens, which has a predetermined refractive power only in the sub-scanning direction. Each of such elements as the aperture stop 52, the collimator lens 53, and the cylindrical lens 54 composes an element of the incidence optical means 62.
Numeral 55 designates a deflecting means, which is comprised, for example, of a rotary polygon mirror and which is rotated at a constant speed in a direction of an arrow A in the drawing by a driving means such as a motor or the like (not illustrated). Numeral 56 denotes a scanning optical means having the f-xcex8 characteristic, which has two f-xcex8 lenses as first and second f-xcex8 lenses. The scanning optical means 56 establishes a conjugate relation between the vicinity of a deflecting facet 55a of the optical deflector 55 and the vicinity of the photosensitive drum surface 57 as a surface to be scanned, in the sub-scanning section, thus having an inclination correction function.
Numeral 58 indicates a return mirror (which will be referred to hereinafter as a xe2x80x9cBD mirrorxe2x80x9d), which reflects a plurality of beams (BD beams) for detection of synchronous signals for adjusting the timing of scan start positions on the photosensitive drum surface 57, toward a synchronism detector 61 described hereinafter. Numeral 59 represents a slit plate (hereinafter referred to as a xe2x80x9cBD slit platexe2x80x9d), which is located at a position equivalent to the photosensitive drum surface 57. Numeral 60 denotes an imaging lens (hereinafter referred to as a xe2x80x9cBD lensxe2x80x9d), which is mounted for establishing a conjugate relation between the BD mirror 58 and the synchronism detector 61 and which corrects surface inclination of the BD mirror 58. Numeral 61 designates a photosensor as a synchronism detector (which will be referred to hereinafter as a xe2x80x9cBD sensorxe2x80x9d). Each of such elements as the return mirror 58, the, BD slit plate 59, the BD lens 60, and the BD sensor 61 constitutes an element of a synchronism-detecting optical means (or BD optical system).
In the same figure, BD detection is carried out for each of the BD beams and the timing of the scan start position for image recording onto the photosensitive drum surface 57 is adjusted for each of the BD beams by use of-output from the BD sensor 61.
Incidentally, in the case of the multi-beam scanning optical systems with a plurality of light-emitting regions (light sources), if the spacing in the main scanning direction between the light sources varies with progress in scanning for various reasons, it will result in deteriorating a printed image. The printed image will also deteriorate if there is deviation between writing start positions of the respective light-emitting regions, even without the variation in the spacing in the main scanning direction between the light-emitting regions during scanning.
A cause to induce the above phenomenon is conceivably existence of difference between defocus amount of the BD beams on the BD slit surface and defocus amount of the scanning beams on the surface to be scanned.
This will be explained below with reference to FIGS. 16A and 16B to FIGS. 21A, 21B, and 21C. It is noted that marginal rays are omitted in FIG. 17A, FIG. 18A, FIG. 20A, and FIG. 21A in order to avoid complication of illustrations.
FIG. 16A shows a state in which each of beams (A- and B-beams in this case) is focused just at one edge on the BD slit plate in the main scanning direction. The A-beam scans the slit plate from left to right in the drawing and first enters the BD sensor just at the left edge of a slit in the BD slit plate, whereupon the BD sensor outputs a signal to indicate the entrance of the A-beam. The B-beam also scans the slit plate from left to right and, just as the A-beam, it first enters the BD sensor just at the left edge of a slit in the BD slit plate, whereupon the BD sensor outputs a signal to indicate the entrance of the B-beam. The timing of the writing start positions of the A- and B-beams is adjusted by detecting the timing of these two signals.
However, if the focus position in the main scanning section of the A- and B-beams having passed through the BD optical system is shifted by xcex4M to this side, i.e., toward the deflecting means as illustrated in FIG. 17A, there will occur the phenomenon as described below, so as to cause the difference between start positions of the A- and B-beams. The A-beam without defocus (actual A-beam) is converged at the left edge of the slit in the BD slit plate and is about to enter the BD sensor at this point. In contrast, the A-beam with defocus (original A-beam) has already entered the surface of the BD sensor (the right dashed line in the figure). The A-beam actually starts entering the BD sensor when arriving at the position of the left solid line in the figure. Therefore, the start timing of the A-beam is earlier by the degree of deviation between the dashed line and the solid line. On the other hand, the B-beam (original B-beam) fails to enter the sensor because of the defocus though it should start entering the BD sensor at the left dashed line. Actually, the B-beam can first enter the BD sensor at the position of the right solid line (actual B-beam) and thus the start timing of the B-beam becomes later by the deviation between the left dashed line and the right solid line. As a result, the start positions of the A- and B-beams will have a difference equal to the distance between the two dashed lines on the BD slit surface.
The difference xcex4Y between the start positions of the A- and B-beams is determined by the defocus amount xcex4M and the angle of incidence xcex8 [rad] (the angle of incidence being 0 [rad] when the beam is incident in parallel to the optical axis of the BD optical system) and can approximately be described as follows.
xcex4Y=xcex4Mxc3x97xcex8xe2x80x83xe2x80x83(1) 
Similarly, a maximum difference xcex4Ytotal between the start positions of the respective light-emitting regions is determined as follows where xcex8max [rad] indicates a maximum angle difference between angles of incidence.
xcex4Ytotal=xcex4Mxc3x97xcex8maxxe2x80x83xe2x80x83(2) 
Therefore, where xcex4Ymax represents a permissible maximum difference between start positions of respective scan lines and xcex4Mmax a permissible maximum defocus determined from xcex4Ymax, the multi-beam scanning optical system needs to be constructed so that the defocus amount xcex4M satisfies the following relation.
|xcex4Mxe2x89xa6xcex4Mmax=xcex4Ymax/xcex8maxxe2x80x83xe2x80x83(3) 
It is preferable that xcex4Ymax be not more than about half of the resolution in the sub-scanning direction. Over this range, adjacent lateral lines will start looking as shifted from each other and the result of printing will become very hard to look.
In this connection, supposing xcex4Ymax=10 xcexcm (which is equal to a half dot in the density of 1200 dpi) and xcex8max=0.5 [rad], the maximum defocus should be as follows.
xcex4Mmax=1.15 mm 
However, above Equations (1) to (3) hold when only the BD optical system is out of focus. If defocus also occurs on the surface to be scanned, in the same amount and in the same direction (toward the deflecting means in FIGS. 17A and 17B) as that of the BD optical system, the difference between start positions of the A- and B-beams, which is called a dot shift, will rarely occur. Let us suppose that the focus position in the main scanning section of the A- and B-beams having passed through the BD optical system is shifted by xcex4M to this side, i.e., toward the deflecting means as illustrated in FIG. 18A. At this time the dot shift will appear between the A- and B-beams as described previously. However, if there is uniform defocus of xcex4M on the surface to be scanned, the dot shift will appear at the position xcex4M apart as illustrated in FIG. 18B.
However, the ideal image plane (the surface to be scanned) is placed xcex4M apart on that side and the distance between the two A- and B-beams becomes almost zero when the A- and B-beams are incident to the ideal image plane (the surface to be scanned). FIG. 18C is an illustration of the positional relation between rays near the axis and it is also seen that the dot shift is canceled out, from the fact that there is a relation close to congruence between a triangle having oblique lines along the principal rays of the A- and B-beams and a base along a straight line at the position xcex4M apart from the surface to be scanned, and a triangle having oblique lines along the original A- and B-beams indicated by the dashed lines in FIG. 18A and a base along the BD slit surface. In this case, however, the position on the surface to be scanned deviates from the best spot position, but the image quality is rarely affected thereby as long as the focus position is within the permissible depth range.
The above described the case in which the focus position is shifted to this side of the BD slit plate, but the same can also apply to the case in which the focus position is shifted to that side of the BD slit plate, as seen from FIGS. 19A and 19B to FIGS. 21A, 21B, and 21C.
From reverse observation of the above description, it is seen-that, where there is uniform defocus on the surface to be scanned, the image will deteriorate unless the focus position in the BD optical system also deviates similarly. When the defocus amount on the surface to be scanned and the defocus amount in the BD optical system are normalized separately from each other, it can be expected that a great dot shift will appear if the defocuses are opposite to each other. Further, it is also readily predictable that if there is great curvature of field on the surface to be scanned the spacing between the beams will also vary according to the curvature. It is thus seen that the multi-beam scanning optical system needs to be constructed so that the defocus amount xcex4X at each image height on the surface to be scanned satisfies the following relation from Eq. (3).
|xcex4Xxe2x88x92xcex4M|xe2x89xa6xcex4Mmax=xcex4Ymax/xcex8maxxe2x80x83xe2x80x83(4) 
Of course, if angles of incidence of the respective beams to the surface to be scanned are exactly equal, the start positions of scan lines will deviate uniformly equal and there will occur no deviation between the start positions of the respective light-emitting regions, because the start positions simply deviate all together.
However, the condition as described above cannot be realized except in the case where the light-emitting regions are placed in a state without deviation in the main scanning direction, i.e., are aligned in a line in the sub-scanning direction, or except in the case where the principal rays of the respective beams are crossed on the polygon mirror surface by use of a relay optical system. In the former case, where the light-emitting regions are arranged in this way and, particularly, where the system is constructed as an enlarging system in the sub-scanning direction, the distance between the light-emitting regions is normally too short, approximately from several xcexcm to several ten xcexcm (whereas the distance is normally about 100 xcexcm between the light-emitting regions of multiple lasers commercially available), and this will cause crosstalk and make a difference between light amounts of the respective light-emitting regions to impede stable oscillation, and will further tend to shorten the lifetime. In the latter case, use of the relay optical system increases the number of necessary optical elements and thus is not preferable in terms of the space and cost.
In the case of the multi-beam scanning optical systems without the BD slit in the BD optical system, the edge portions of the BD sensor result in functioning as the BD slit and thus the above description can be understood by replacing the left edge of the slit in the BD slit plate with the left edge of the effective part of the BD sensor and the BD slit surface with a photoreceptive surface of the BD sensor.
The scanning direction was from left to right in the drawing in the above description, but the same can also apply to the case of the opposite scanning direction except that the left edge of the slit in the BD slit plate to determine the timing of writing start is replaced by the right edge of the slit in the BD slit plate on the right side, which is not illustrated in the figures.
An object of the present invention is to provide a multi-beam scanning optical system capable of implementing high-quality printing in relatively simple structure and at high speed, and an image forming apparatus using it.
In the multi-beam scanning optical system designed to perform the BD detection for each of the BD beams as illustrated in FIG. 22, where the focus position of the BD beams deviates from the BD slit surface because of manufacturing errors of lenses, assembly errors, focus errors of lenses, and so on, except in the case where the beams are incident in parallel to the optical axis of the BD optical system (the beams will be represented by the principal rays of the respective beams in the following description, because the beams with width complicate understanding of the timing of BD), the timing when the principal rays of the respective BD beams pass the edge of the BD slit, will differ from that without the defocus, posing the problem that the start positions of images deviate from each other.
FIGS. 29A, 29B, and 29C are principal, schematic diagrams to show the positional relation between the principal rays of partial beams (BD beams) of the respective beams emitted from the two light-emitting regions (light sources). FIG. 29A shows the positional relation between the principal rays from the respective light-emitting regions in the ideal state of the BD beams without defocus, FIG. 29B the positional relation between the principal rays from the respective light-emitting regions in the case of occurrence of defocus of the BD beams, and FIG. 29C the positional relation between the principal rays from the respective light-emitting regions with improvement in the converging state on the BD slit surface by some method.
The writing start positions on the surface to be scanned are originally timed by the beams going past by one edge of the BD slit as illustrated in FIG. 29A, whereas, with occurrence of defocus of the BD beams as illustrated in FIG. 29B, the original beam (indicated by the solid line in the same figure) is intercepted by the BD slit plate in the case of the A-beam and the actual start position of the A-beam is determined by the ray indicated by the dashed line. In other words, the start position of the A-beam is shifted by the degree of transition from the state of the solid line to the state of the dashed line of the A-beam. The same can also apply to the B-beam, so that writing thereof starts earlier by the deviation between the solid line and the dashed line. Therefore, the start positions of the A- and B-beams will deviate from each other by the distance between the solid lines on the BD slit surface.
The deviation amount xcex4Y between the start positions of the A- and B-beams is determined from the defocus amount xcex4M and the angle of incidence xcex8 (the angle of incidence being 0xc2x0 in the parallel state to the optical axis of the BD optical system), and can approximately be described as follows.
xcex4Y=xcex4Mxc2x7tan(xcex8)xe2x80x83xe2x80x83(5) 
Similarly, the total deviation xcex4Ytotal is determined as follows where the maximum angle difference between angles of incidence is xcex8max.
xcex4Ytotal=xcex4Mxc2x7tan(xcex8max)xe2x80x83xe2x80x83(6) 
Letting xcex4Ymax be the permissible maximum deviation between start positions of scan lines and xcex4max be the permissible maximum defocus determined from xcex4Ymax, the multi-beam scanning optical system needs to be constructed so that the defocus amount xcex4M satisfies the following relation.
|xcex4M|xe2x89xa6xcex4Mmax=xcex4Ymax/tan(xcex8max)xe2x80x83xe2x80x83(7) 
For example, supposing xcex4Ymax=11 xcexcm and xcex8max=0.5xc2x0, xcex4Mmax=1.26 mm.
Of course, if the angles of incidence of the respective beams onto the surface to be scanned are exactly equal, the start positions of the scan lines will deviate uniformly equal and there will appear no deviation between the write start positions of the respective light-emitting regions, because the start positions simply deviate all together.
A multi-beam scanning optical system according to one aspect of the present invention is a multi-beam scanning optical system comprising incidence optical means for guiding a plurality of beams emitted from light source means having a plurality of light-emitting regions spaced apart from each other in a main scanning direction, to deflecting means; scanning optical means for focusing the plurality of beams deflected by the deflecting means, on a surface to be scanned, to form a plurality of scan lines; and synchronism-detecting optical means for converging part of the plurality of beams deflected by the deflecting means, on a slit surface by a lens section, thereafter guiding the beams to a synchronism detector, and controlling timing of a scan start position on the surface to be scanned for each of the plurality of beams by use of a signal from the synchronism detector,
wherein the following condition is satisfied:
|xcex4M1|xe2x89xa6xcex4Ymax/tan(xcex8max) 
(where
xcex4M1: defocus amount in a main scanning section of the beams guided to the synchronism detector and in a view from the slit;
xcex4Ymax: permissible dot shift amount per scan line;
xcex8max: maximum angle difference between angles of incidence to the slit surface of the beams used for detection of synchronism).
In the multi-beam scanning optical system according to another aspect of the invention, the permissible dot shift amount per scan line is not more than half of resolution in a sub-scanning direction.
The multi-beam scanning optical system according to another aspect of the invention comprises correction means for relatively shifting a focus position in the main scanning section of the beams guided to said synchronism detector in a direction of the optical axis of said synchronism-detecting optical means from said slit surface.
The multi-beam scanning optical system according to another aspect of the invention comprises correction means for moving the position of said slit surface or a unit including the slit surface in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, said lens section is disposed in an optical path between said deflecting means and said slit surface, and the optical system comprises correction means for moving said lens section in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, at least one lens forming said lens section is integrated with said scanning optical means, and the optical system comprises correction means for moving at least one lens of the lens section not integrated with the scanning optical means, and said slit surface in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, said lens section is integrated with said scanning optical means, and the optical system comprises correction means for moving at least one optical element of the scanning optical means in a direction of the optical axis of the scanning optical means and for moving said slit surface in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, at least one lens forming said lens section is integrated with said scanning optical means, and the optical system comprises correction means for moving at least one lens forming the scanning optical means in the main scanning direction.
A multi-beam scanning optical system according to a further aspect of the invention is a multi-beam scanning optical system comprising incidence optical means for guiding a plurality of beams emitted from light source means having a plurality of light-emitting regions spaced apart from each other in a main scanning direction, to deflecting means; scanning optical means for focusing the plurality of beams deflected by the deflecting means, on a surface to be scanned, to form a plurality of scan lines; and synchronism-detecting optical means for converging part of the plurality of beams deflected by the deflecting means, on a slit surface by a lens section, thereafter guiding the beams to a synchronism detector, and controlling timing of a scan start position on the surface to be scanned for each of the plurality of beams by use of a signal from the synchronism detector,
said multi-beam scanning optical system comprising correction means for correcting a dot shift per scan line on the surface to be scanned, which occurs because of a defocus amount xcex4M1 in a main scanning section of the beams guided to the synchronism detector and in a view from the slit surface.
In the multi-beam scanning optical system according to another aspect of the invention, said dot shift is not more than half of resolution in a sub-scanning direction.
In the multi-beam scanning optical system according to another aspect of the invention, said plurality of light-emitting regions are spaced apart from each other in the main scanning direction and in the sub-scanning direction.
In the multi-beam scanning optical system according to another aspect of the invention, a slit in said slit surface is inclined in the sub-scanning direction according to the dot shift per scan line on said surface to be scanned.
The multi-beam scanning optical system according to another aspect of the invention comprises rotating means for rotating said slit surface or a unit including the slit surface about the optical axis of the synchronism-detecting optical means according to the dot shift per scan line on said surface to be scanned.
A multi-beam scanning optical system according to a further aspect of the invention is a multi-beam scanning optical system comprising incidence optical means for guiding a plurality of beams emitted from light source means having a plurality of light-emitting regions spaced apart from each other in a main scanning direction, to deflecting means; scanning optical means for focusing the plurality of beams deflected by the deflecting means on a surface to be scanned, to form a plurality of scan lines; and synchronism-detecting optical means for guiding part of the plurality of beams deflected by the deflecting means, to a synchronism detector by a lens section and controlling timing of a scan start position on the surface to be scanned for each of the plurality of beams by use of a signal from the synchronism detector,
wherein the following condition is satisfied:
|xcex4M2|xe2x89xa6xcex4Ymax/tan(xcex8max) 
(where
xcex4M2: defocus amount in a main scanning section of the beams guided to the synchronism detector and in a view from a photoreceptive surface of the synchronism detector;
xcex4Ymax: permissible dot shift amount per scan line;
xcex8max: maximum angle difference between angles of incidence to the photoreceptive surface of the beams used for detection of synchronism).
In the multi-beam scanning optical system according to another aspect of the invention, the permissible dot shift amount per scan line is not more than half of resolution in a sub-scanning direction.
The multi-beam scanning optical system according to another aspect of the invention comprises correction means for relatively shifting a focus position in the main scanning section of the beams guided to said synchronism detector in a direction of the optical axis of said synchronism-detecting optical means from the photoreceptive surface of the synchronism detector.
The multi-beam scanning optical system according to another aspect of the invention comprises correction means for moving the position of said synchronism detector or a unit including the synchronism detector in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, said lens section is disposed in an optical path between said deflecting means and said synchronism detector, and the optical system comprises correction means for moving said lens section in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, at least one lens forming said lens section is integrated with said scanning optical means, and the optical system comprises correction means for moving at least one lens of the lens section not integrated with the scanning optical means, and said synchronism detector in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, said lens section is integrated with said scanning optical means, and the optical system comprises correction means for moving at least one optical element of the scanning optical means in a direction of the optical axis of the scanning optical means and for moving said synchronism detector in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, at least one lens forming said lens section is integrated with said scanning optical means, and the optical system comprises correction means for moving at least one lens forming the scanning optical means in the main scanning direction.
A multi-beam scanning optical system according to a further aspect of the invention is a multi-beam scanning optical system comprising incidence optical means for guiding a plurality of beams emitted from light source means having a plurality of light-emitting regions spaced apart from each other in a main scanning direction, to deflecting means; scanning optical means for focusing the plurality of beams deflected by the deflecting means, on a surface to be scanned; and synchronism-detecting optical means for converging part of the plurality of beams deflected by the deflecting means, on a slit surface by a lens section, thereafter guiding the beams to a synchronism detector, and controlling timing of a scan start position on the surface to be scanned by use of a signal from the synchronism detector,
wherein, where xcex4M1 is a defocus amount in a main scanning section of the beams guided to the synchronism detector and in a view from the slit surface and xcex4X is a defocus amount at each image height on the surface to be scanned, the following condition is satisfied:
|xcex4Xxe2x88x92xcex4M1|xe2x89xa6xcex4Ymax/xcex8max 
(where
xcex4Ymax: permissible dot shift amount per scan line;
xcex8max: maximum angle difference between angles of incidence to the slit surface of the beams used for detection of synchronism).
In the multi-beam scanning optical system according to another aspect of the invention, the permissible dot shift amount per scan line is not more than half of resolution in a sub-scanning direction.
The multi-beam scanning optical system according to another aspect of the invention comprises correction means for relatively shifting a focus position in the main scanning section of the beams guided to said synchronism detector in a direction of the optical axis of said synchronism-detecting optical means from said slit surface.
The multi-beam scanning optical system according to another aspect of the invention comprises correction means for moving the position of said slit surface or a unit including the slit surface in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, said lens section is disposed in an optical path between said deflecting means and said slit surface, and the optical system comprises correction means for moving said lens section in a direction of the optical axis of said synchronism-detecting optical means.
A multi-beam scanning optical system according to a further aspect of the invention is a multi-beam scanning optical system comprising incidence optical means for guiding a plurality of beams emitted from light source means having a plurality of light-emitting regions spaced apart from each other in a main scanning direction, to deflecting means; scanning optical means for focusing the plurality of beams deflected by the deflecting means, on a surface to be scanned, to form a plurality of scan lines; and synchronism-detecting optical means for converging part of the plurality of beams deflected by the deflecting means, on a slit surface by a lens section, thereafter guiding the beams to a synchronism detector, and controlling timing of a scan start position on the surface to be scanned for each of the plurality of beams by use of a signal from the synchronism detector;
where xcex4M1 is a defocus amount in a main scanning section of the beams guided to the synchronism detector and in a view from the slit surface and xcex4X is a defocus amount at each image height on the surface to be scanned, said multi-beam scanning optical system comprising correction means for correcting a dot shift per scan line on the surface to be scanned, which occurs because of a difference between the two defocus amounts xcex4M1, xcex4X.
In the multi-beam scanning optical system according to another aspect of the invention, said dot shift is not more than half of resolution in a sub-scanning direction.
In the multi-beam scanning optical system according to another aspect of the invention, said plurality of light-emitting regions are spaced apart from each other in the main scanning direction and in the sub-scanning direction.
In the multi-beam scanning optical system according to another aspect of the invention, a slit in said slit surface is inclined in the sub-scanning direction according to the dot shift per scan line on said surface to be scanned.
The multi-beam scanning optical system according to another aspect of the invention comprises rotating means for rotating said slit surface or a unit including the slit surface about the optical axis of the synchronism-detecting optical means according to the dot shift per scan line on said surface to be scanned.
A multi-beam scanning optical system according to a further aspect of the invention is a multi-beam scanning optical system comprising incidence optical means for guiding a plurality of beams emitted from light source means having a plurality of light-emitting regions spaced apart from each other in a main scanning direction, to deflecting means; scanning optical means for focusing the plurality of beams deflected by the deflecting means, on a surface to be scanned; and synchronism-detecting optical means for guiding part of the plurality of beams deflected by the deflecting means, to a synchronism detector by a lens section and controlling timing of a scan start position on the surface to be scanned by use of a signal from the synchronism detector,
wherein, where xcex4M2 is a defocus amount in a main scanning section of the beams guided to the synchronism detector and in a view from a photoreceptive surface of said synchronism detector and xcex4X is a defocus amount at each image height on the surface to be scanned, the following condition is satisfied:
|xcex4Xxe2x88x92xcex4M2|xe2x89xa6xcex4Ymax/xcex8max 
(where
xcex4Ymax: permissible dot shift amount per scan line;
xcex4max: maximum angle difference between angles of incidence to the photoreceptive surface of the beams used for detection of synchronism).
In the multi-beam scanning optical system according to another aspect of the invention, the permissible dot shift amount per scan line is not more than half of resolution in a sub-scanning direction.
The multi-beam scanning optical system according to another aspect of the invention comprises correction means for relatively shifting a focus position in the main scanning direction of the beams guided to said synchronism detector in a direction of the optical axis of said synchronism-detecting optical means from the photoreceptive surface of the synchronism detector.
The multi-beam scanning optical system according to another aspect of the invention comprises correction means for moving the position of said synchronism detector or a unit including the synchronism detector in a direction of the optical axis of said synchronism-detecting optical means.
In the multi-beam scanning optical system according to another aspect of the invention, said lens section is disposed in an optical path between said deflecting means and said synchronism detector, and the optical system comprises correction means for moving said lens section in a direction of the optical axis of said synchronism-detecting optical means.
An image forming apparatus according to one aspect of the invention is an image forming apparatus comprising the multi-beam scanning optical system as described above; a photosensitive member placed on said surface to be scanned; a developing unit for developing an electrostatic latent image formed on said photosensitive member with scanning light by said multi-beam scanning optical system, into a toner image; a transfer unit for transferring said developed toner image onto a transfer medium; and a fixing unit for fixing the transferred toner image on the transfer medium.
An image forming apparatus according to another aspect of the invention is an image forming apparatus comprising the multi-beam scanning optical system as described above; and a printer controller for converting code data supplied from an external device, into an image signal and entering the image signal into said multi-beam scanning optical system.